In an ink jet printer, a print is made by ejecting or jetting a series of small droplets of ink onto a paper to form picture elements (pixels) in an image-wise pattern. The density of a pixel is determined by the amount of ink jetted onto an area. Control of pixel density is generally achieved by controlling the number of droplets of ink jetted into an area of the print. To produce a print containing a single color, for example a black and white print, it is only necessary to jet a single black ink so that more droplets are directed at areas of higher density than areas with lower density.
Color prints are generally made by jetting, in register, inks corresponding to the subtractive primary colors cyan, magenta, yellow, and black. In addition, specialty inks can also be jetted to enhance the characteristics of a print. For example, custom colors to expand the color gamut, low density inks to expand the gray scale, and protective inks such as those containing UV absorbers can also be jetted to onto a paper to form a print.
Ink jet inks are generally jetted onto the paper using a jetting head. Such heads can jet continuously using a continuously jetting print head, with ink jetted towards unmarked or low density areas deflected into a gutter and recycled back into the ink reservoir. Alternatively, ink can be jetted only where it is to be deposited onto the paper using a so-called drop on demand print head. Commonly used heads eject or jet droplets of ink using either heat (a thermal print head) or a piezoelectric pulse (a piezoelectric print head) to generate the pressure on the ink in a nozzle of the print head to cause the ink to fracture into a droplet and eject from the nozzle.
Ink jet printers can broadly be classified as serving one of two markets. The first is the consumer market, where printers are slow; typically printing a few pages per minute and the number of pages printed is low. The second market consists of commercial printers, where speeds are typically at least hundreds of pages per minute for cut sheet printers and hundreds of feet per minute for web printers. For use in the commercial market, ink jet prints must be actively dried as the speed of the printers precludes the ability to allow the prints to dry without specific drying subsystems.
FIG. 1 is a system diagram of one example of a prior art commercial printing system 2. In the example of FIG. 1, commercial printing system 2 has a supply 4 of a paper 6 and a transport system 8 for moving paper 6 past a plurality of printheads 10A, 10B, and 10C. Printheads 10A, 10B and 10C eject ink droplets onto paper 6 as paper 6 is moved past printheads 10A, 10B and 10C by transport system 8. Transport system 8 then moves paper 6 to an output area 14. In this example, paper 6 is shown as a continuous web that is drawn from a spool type supply 4, past printheads 10A, 10B and 10C to an output area 14 where the printed web is wound on to a spool 18. In the embodiment illustrated here, transport system 8 comprises a motor that rotates spool 18 to pull paper 6 past printheads 10A, 10B and 10C.
Inkjet inks generally comprise up to about 97% water or another jettable carrier fluid such as an alcohol that carries colorants such as dyes or pigments dissolved or suspended therein to the paper. Ink jet inks also conventionally include other materials such as humectants, biocides, surfactants, and dispersants. Protective materials such as UV absorbers and abrasion resistant materials may also be present in the inkjet inks. Any of these may be in a liquid form or may be delivered by means of a liquid carrier or solvent. Conventionally, these liquids are selected to quickly vaporize after printing so that a pattern of dry colorants and other materials forms on the receiver soon after jetting.
Commercial inkjet printers typically print at rates of more than fifty feet of printing per minute. This requires printheads 10A, 10B and 10C to eject millions of droplets 12A, 12B and 12C of inkjet ink per minute. Accordingly, substantial volumes of liquids are ejected and begin evaporating at each of printheads 10A, 10B and 10C during operation of such printers.
When an ink jet image is printed on an absorbent paper, the inkjet ink droplets penetrate and are rapidly absorbed by the paper. As the ink is absorbed into the paper, the carrier fluid in the ink droplets spread colorants. A certain extent of spreading is anticipated and this spreading achieves the beneficial effect of increasing the extent of a surface area of the paper colored by the inkjet ink color. However, where spreading exceeds an expected extent, printed images can exhibit any or all of a loss of resolution, a decrease in color saturation, a decrease in density or image artifacts created by unintended combinations of colorants.
Absorption of the carrier fluid from inkjet inks can also have the effect of modifying the dimensional stability of an absorbent paper. In this regard it will be appreciated that the process of paper fabrication creates stresses in the paper that are balanced to create a flat paper stock. However, wetting of the paper partially or completely releases such stresses. In response, the paper cockles and distorts creating significant difficulties during subsequent paper handling, printing, or finishing applications. Cockle and distortion can reduce color to color registration, color saturation, and print density. In addition, cockle and distortion of a print can impede the ability of a printing system to print front and back sides of a paper in register, often referred to as justification.
Further, in some situations, the jetting of large amounts of inkjet ink onto an absorbent paper can reduce the web strength of the paper. This can be particularly problematic in printers such as inkjet printing system 2 that is illustrated in FIG. 1, where, paper 6 is advanced by pulling the paper as the pulling applies additional external stresses to the paper that can further distort the paper.
Semi-absorbent papers absorb the ink more slowly than do absorbent papers. Inkjet printing on semi-absorbent papers can cause liquids from the inkjet ink to remain in liquid form on a surface of the paper for a period of time. Such ink is subject to smearing and offsetting if another surface contacts the printed surface before the carrier fluid in the ink evaporates. Air flow caused by either a drying process or by the transport of the receiver can also distort the wet print. Finally, external contaminants such as dust or dirt can adhere to the wet ink, resulting in image degradation.
To avoid these effects, high speed inkjet printed papers are frequently actively dried using one or more dryers such as dryers 16A, 16B and 16C shown in FIG. 1. Dryers 16A, 16B and 16C typically heat the printed paper and ink, to increase the evaporation rate of carrier fluid from paper 6 in order to reduce drying times. As is shown in FIG. 1, dryers 16A, 16B and 16C are typically positioned as close to the jetting assembly as possible so that the ink is dried in as short a time as possible after being jetted onto the paper. Indeed, it would be desirable to position the dryer subsystem in the vicinity of the jetting module.
However, the increased the rate at which carrier fluid evaporates creates localized concentrations of vaporized carrier fluid 17 around printing heads 10A, 10B and 10C. Further, movement of paper 6 through printer 2 drags air and carrier fluid along with paper 6 forming an envelope of air with carrier fluid vapor therein that travels along with printed paper 6 as printed paper 6 moves from print head 10A, to printhead 10B and on to printhead 10C. Accordingly, when a printed portion of paper 6 reaches second printing area 10B a second inkjet image is printed and dried, the concentration of carrier fluid vapor in the air between second printhead 10B and paper 6 is further increased. A similar result occurs at printhead 10C.
These concentrations increase the probability that vaporized carrier fluids 17 will condense on structures within the printer that are at temperature that is below a condensation point of the evaporated carrier fluid. Such condensation can create electrical shorts, cause corrosion and can interfere with ink jet droplet formation. Further, there is the risk that such condensates will form droplets 19 on structures such as printhead 10B or printhead 10C from which they can fall, transfer or otherwise come into contact with a printed paper so as to create image artifacts on the paper. This risk is particularly acute for structures that are in close proximity to a paper path through the printer.
One particular risk is the risk of problems created by such condensates at the inkjet printheads. When condensates form in such locations the condensates can combine with carrier fluid in ink droplets jetted toward a receiver to create image artifacts and can also interfere with droplet formation and negatively influence the flight path taken by the droplets. Accordingly, it is desirable to provide some level of protection against the formation of such drops of condensation at the printhead.
It will also be appreciated that it is frequently the case that several printheads are used in proximity to form what is known in the art as a printing module or linehead. Concentrations of vaporized carrier fluid can vary significantly at different printheads in the printing module. Accordingly, what is also needed is an ability to provide condensation protection for a plurality of printheads and to do so in a manner that allows for individualized adjustment at the printheads.